1. Field of the Invention
The present invention relates to an image recording apparatus for recording an image by using a recording head in which a plurality of recording devices are arranged.
2. Related Background Art
With wide use of information processing equipment such as copying machines, word processors, computers and the like and communicating equipment, digital image recording apparatuses for recording images by an ink jet recording method or a thermal transferring method have been rapidly widely used as an image forming (recording) apparatuses for the above-described equipment. The recording apparatus of the type described above usually employs a recording head (hereinafter called a "multi-head") in which a plurality of recording elements are integrally disposed in order to raise the recording speed.
For example, an ink jet recording head usually comprises a so-called "multi-nozzle head" in which a plurality of ink orifices and liquid passages are integrally provided. A thermal head arranged to act in accordance with a thermal transfer method or a heat sensitive method usually comprises a plurality of heaters integrally provided therein.
However, it is difficult to uniformly manufacture the recording elements of the multi-head due to non-uniform characteristics generated in the manufacturing process or non-uniform characteristics of the material of the head. Therefore, the recording elements involve non-uniform characteristics to a certain degree. For example, the above-described multi-nozzle head encounters a non-uniform shape of its orifice or liquid passage. On the other hand, the thermal head encounters a problem of non-uniform shape or resistance of its heater. The non-uniformity of the characteristics between the recording elements will cause uneven size or image density of dots to be recorded by each of the recording elements. Consequently, uneven image density is generated in the recorded image.
In order to overcome the above-described problem, a variety of methods have been disclosed each of which is arranged in such a manner that the uneven image density is visually detected or the adjust image is visually inspected and a signal to be supplied to each of the recording elements is manually corrected whereby an even image is obtained.
An ordinary manual correction operation is performed as follows: When an uneven image density as shown in FIG. 44C is visually detected when the same signal is supplied to each of recording elements 331 arranged, as shown in FIG. 44A in a multi-head 330, the input signal is, as shown in FIG. 44D, corrected so as to supply a large input signal to the recording elements in a portion the density of which is low, while a small input signal is supplied to the recording elements in a portion the density of which is high.
In a recording method in which the dot diameter or the dot density can be varied, the diameter of dot to be recorded by each of the recording elements is modified in accordance with the input. As a result, a gradation of recording can be achieved. For example, in an ink jet recording head arranged to act in accordance with a piezoelectric method or a bubble jet method, driving voltage or the width of pulses to be applied to an ejecting energy generating element such as the piezoelectric element or an eletrothermal conversion element is modulated in accordance with the input signal. In the thermal head, the driving voltage or the pulse width to be applied to each of the heaters is modulated in accordance with the same. As a result, it can be considered that the dot diameter or the dot density realized by each of the recording elements can be made uniform. Therefore, the image distribution can be made uniform as shown in FIG. 44E. In a case where it is difficult to modulate the driving voltage or the pulse width or in a case where it is difficult to adjust the image density in a wide range even if the above-described factors are modified, for example in a case where one pixel is constituted by a plurality of dots, the number of dots to be recorded is modified in response to the input signal. As a result, a multiplicity of dots can be recorded in the portion the density of which is low, while a reduced number of dots can be recorded in the portion the density of which is high. In a case where one pixel is constituted by one dot, the dot diameter can be changed by modulating the number of ink ejections (number of ink jetting) with respect to one pixel in an ink jet recording apparatus. As a result, the image density can be made uniform as shown in FIG. 44.
According to a disclosure made by the applicant of the present invention in Japanese Patent Appln. Laid-Open No. 57-41965, a color image is automatically read out by an optical sensor, a correction signal is supplied to ink jet recording heads of the corresponding colors whereby a desired color image is formed. According to this disclosure, a basic automatic adjustment has been disclosed and an important technology has been disclosed. However, a variety of problems arise when the above-described technology is adapted to a various apparatuses. In the above-described disclosure, there is no description about the technological problem which can first be overcome by the present invention.
On the other hand, as disclosed in Japanese Patent Appln. Laid-Open No. 60-206660, U.S. Pat. No. 4,328,504, Japanese Patent Appln. Laid-Open Nos. 50-147241 and 54-27728, a structure has been known which is arranged in such a manner that the position which the droplet has reached is automatically read and it is corrected so as to adjust the position which the droplet reaches. According to any one of the above-described structures, there is no description about the technological problem which can first be overcome by the present invention although a common technology of automatically adjusting the operation has been disclosed.
In order to overcome the above-described problems, it is effective to arrange the image forming apparatus in such a manner that an image density reading portion is provided therein and the uneven image density distribution in a range of the configuration of the recording elements is periodically read so as to remake the uneven image density data. In the structure constituted as described above, the correction data can be properly processed again if the uneven image density distribution of the head is changed. Therefore, a uniform image without unevenness can always be formed.
FIG. 48 illustrates an example of an uneven image density reading unit for use in the above-described method. Referring to FIG. 48, reference numeral 501 represents a recording medium on which a test pattern for measuring the unevenness is formed. Reference numeral 502 represents a light source for irradiating the surface of the recording medium 501. Reference numeral 503 represents a sensor for reading reflected light from the surface of the recording medium 501. Reference numerals 504 and 505 represent lenses and 506 represents a reading unit on which the above-described elements are mounted. The thus constituted reading unit 506 is caused to perform the scanning operation to read the unevenness distribution whereby the unevenness correction data can be remade.
FIG. 49 illustrates another example of the uneven image density reading unit. Referring to FIG. 49, reference numeral 520 represents a line sensor comprising a CCD or the like, 521 represents a reading pixel of the line sensor 520 and 524 represents a correction test pattern in which recording elements are formed in a width d in direction y. Since line sensor 520 and the recording medium relatively move in direction x, the density of the test pattern formed by the recording head is read out. Therefore, data read by each of the pixels 521 of the line sensor 520 corresponds to the density of data formed by each of the recording elements of the recording head.
When the image density is accurately measured or the correction degree is accurately determined, there has been unevenness depending upon the status of the apparatus. There has been a known structure in which an initial adjustment is performed when the recording operation of the apparatus is started for the purpose of obtaining a desired result of the recording operation. However, since the above-described accurate adjustment is performed when it is considered necessary, a satisfactory effect cannot be obtained heretofore.
In particular, the temperature of the recording head is controlled in a predetermined range in order to stabilize its recording characteristics (the ink ejecting characteristics of the ink jet recording head). However, the uneven image density correction suitable for a certain temperature range is not always suitable for another temperature range. Furthermore, if recording of the test pattern at the correction is performed in a state where the recording head cannot act normally, there is a fear that uneven image density in a case where the operation is performed with normal recording characteristics cannot be correctly recognized.
For example, in a case where the unevenness correction data is remade by using a half tone image of a duty of 50%, a satisfactory unevenness correction effect at the 50% half tone can be obtained. However, a satisfactory effect cannot always be obtained at a 30% half tone or 75% half tone. The reason for this lies in that the gradation characteristics of the multi-head are not always linear characteristics as shown in FIG. 46. Therefore, the similar effect cannot be always obtained in the overall area of the input signal by the correcting operation performed as shown FIG. 47.
Therefore, in a case where there is a high image density portion in the original document and unevenness included in this portion is particularly precisely corrected, a sufficient unevenness correction effect cannot be obtained even if unevenness correction data processed with a 50% half tone test pattern is used.
In an ink jet recording operation, ink expands (runs) laterally when ink is absorbed by paper. Therefore, the sub-scanning quantity of paper is made slightly larger than the recording width for one line so as to cover the joints of the lines. However, the quantity of the expansion of the recording width is different depending upon the quantity of recording ink. If a large quantity of recording ink is used, it becomes large, while the same becomes small when a reduced quantity of the recording ink is used. Therefore, in a case where the printing duty is changed as the test pattern for making the unevenness correction data, for example, in a case where the paper sub-scanning quantity is arranged to be the most suitable quantity at the time of the printing duty of 50%, the width of the expansion becomes too large when a test pattern of a duty of 80% is printed. Therefore, the joints of the lines overlap and the density of the joints are raised excessively. If the unevenness correction data is processed by using the above-described pattern, data, which causes a .gamma.-straight line having a small inclination with respect to the end nozzle to be selected, can be processed. When a copying operation is performed by using the above-described data is used, a problem arises in that the end portion of each line becomes too light and the joints generate white linear portions.
When a copying operation is performed by using unevenness correction data processed by a test pattern the printing duty of which is low, a problem arises in that the joints of the lines generate black linear portions. The above-described problems also arise depending upon the type of the recording material.